Conventionally, radio frequency (RF) power amplifiers are used to amplify signals in wireless communication devices such as two-way radios, mobile telephones and satellite transceivers. In the wireless communication devices, batteries typically provide electrical power. Such batteries have limited energy storage capacity. Therefore, reducing power consumption helps to increase an operating period of a wireless communication device as electrical power from batteries is expended less quickly with improved efficiency.
Generally, electrical power in wireless communication devices is consumed mainly by RF power amplifiers. Therefore, improving efficiency of these amplifiers helps to reduce power consumption, for example, in base stations of a wireless communication system and increasing the operating period of handsets communicating with the base stations.
In addition to power consumption, another important parameter that characterises an RF power amplifier is the linearity. Linearity of an RF amplifier is affected by the output of the RF power amplifier. Typically, RF power amplifiers are biased to operate in a saturation mode in order to achieve high efficiency. In the saturation mode, RF power amplifiers typically do not operate as linearly as desired. Consequently, non-linear amplification of an input signal by an RF power amplifier biased to operate in the saturation mode is known to cause signal distortion. Generally, such signal distortion causes negative amplitude and positive phase deviation as RF input signal power increases. Such signal distortion includes what is commonly referred to as intermodulation (IM) distortion as well as harmonic distortion that affect desired signals of a wireless communication device. Furthermore, IM distortion generates undesired signals that interfere with reception or transmission of desired signals of other wireless communication devices.
Linearity problems of an RF amplifier can be alleviated using linearization schemes such as feedback systems and feedforward systems. However, feedback linearization schemes tend to resolve only a narrow bandwidth of RF input signals and therefore cannot efficiently linearize RF amplifiers operating over broader bandwidths. In addition, such linearization schemes typically degrade in performance at margin frequencies of an operation bandwidth compared to frequencies that are nearer to the centre of the operation bandwidth. Furthermore, the feedforward linearization schemes are complicated to implement, as an amplifier's transfer characteristics have to be accurately determined and this requires additional circuitry with more components. Accommodating these additional components is a problem because space is limited in wireless communication devices. Furthermore, controlling the magnitude and phase to cancel or at least reduce signal distortion is not easy.
Another linearization scheme alleviates signal distortion by predistorting a signal such that the signal is complementarily compensated when subjected to non-linear amplification by an RF amplifier. This linearization scheme is known as predistortion and circuitry that effects the predistortion is commonly referred to as a predistortion linearizer. An RF input signal is predistorted by the predistortion linearizer to complement distortion characteristics of the RF power amplifier prior to amplifying by that RF power amplifier. Predistortion linearizers using, for example, diode or GaAs field-effect-transistors (FETs) only predistort an RF input signal at a specific level of input power at the operation frequency of the RF input signal. As such, predistortion linearizers have limited applicability in RF amplifiers that are operable across the entire input power dynamic range in wide bandwidth applications.
Therefore, in view of the above limitations in linearization schemes and circuits, what is needed is a predistortion linearizer to enable an RF amplifier to operate more linearly and with better efficiency across the entire input power dynamic range in wide bandwidth applications.